International Symposium on
Drylands Ecology and Human Security

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From the Micro-topography Organization to Runoff Modeling and to Water Reservoir Management

Jean Duchesne1, Pierre Ozer2, Jean Gassani3

1 Department of Landscape Science, I.N.H., France
Presenting Author: Prof. Dr. Jean Duchesne

2 Department of Environment Sciences and Management, University of Liege, Belgium

3 University of Angers, France


In most theories in hydrology, water which arrives at the river is modeled by the combination of two functions:
i) a production function which transforms the total rainfall into the effective rainfall;
ii) a transfer function which transforms the effective precipitation falling at a given time overall the basin into a time distribution of the discharge at the outlet.

The general idea is to develop a new production function topographically-based and without any calibration. As we assume it is possible to build such a production function by taking into account both the infiltration law deduced from measurements and the organization of micro-topography; the latter often (always?) shows a great number of micro-basins which are linked by superficial micro-talwegs which in their turn converge to the hydrographic network by following the general slope.

We first applied these hypotheses in Indonesia, on very small catchments (10 to 60 hectares).
Here, we propose to verify this hydrological law in southern Mauritania characterized by arid conditions where the studied basin soils are totally bare at the beginning of the rainy season (July). Furthermore, soils are frequently very impervious in such a way that when the water concentrates into micro-basins either it is stocked and evaporates or, if the water level increases sufficiently, it discharges over the natural threshold.
Here, our study area is the basin of Male which extends over 800 km².
This study verifies two scientific objectives:
i) to obtain a general law of the size distribution of micro-basins which are supposed to follow a simple statistical law;
ii) to model the water flux which discharges over the micro-basin thresholds towards the hydrographic network.
This approach is particularly important in arid and semi-arid climates because of the great variability of both annual rainfall and precipitations intensities. It appears that for similar total annual rainfall for two successive years, a given dam-reservoir can or not fill: it only depends on the intensity of main rains.
So the final technical goal is to simulate the discharge of rivers (vs. oueds) in function of time in order to manage the water reservoirs.